Various proposals have beem made heretofore for a multiple sheave stringing block useful in the installation of bundle type power conductors along a power line and including provision for threading the blocks with a helicopter dispensed tow-line. The use of helicopters for the the threading operation requires suitable provision for introducing the two-line into the throat of the stringing block. More pertinent prior teachings known to this applicant and intended to perform this function include: U.S. Pat. Nos. Reilly 3,584,837; Chadwick 3,837,623; Lindsey 3,868,089 Chadwick 3,905,581; Bozeman 4,018,422 and Lindsey 4,129,287. Reilly is typical of blocks employed in stringing bundle conductors and requiring manual threading of individual blocks by a workman laboriously climbing the tower and lowering himself to the throat of each block in order to perform the threading operation by hand. The earlier Chadwick patent discloses a bundle conductor block threadable by helicopter and relies upon a single structural steel side frame to support the three sheaves and employs a tow-line guide member movable to its retracted position in only one direction. Lindsey U.S. Pat. No. 3,868,089 avoids this last mentioned shortcoming of Chadwick but employs heavy duty cast sideframes and a heavy duty load sharing gate which closes across the towline threading passage to distribute the load forces between the two sideframe members after the block is threaded. Chadwick U.S. Pat. No. 3,905,581 provides a helicopter threadable stringing block having a rotary wheel device for transferring the tow-line across the frame while continuously distributing the load forces between the blocks' two sideframes. Bozeman is generally similar in principle but differs structurally from the last mentioned Chadwick patent. The two Lindsey patents are closely related to one another but are readily distinguishable structurally and in principle from this invention.